A polymer electrolyte fuel cell comprises a polymer electrolyte membrane displaying proton permeability and a porous catalyst-electrode provided on both sides of the membrane. Air (or oxygen) and hydrogen are respectively supplied to electrodes and power generation is performed as a result of electrochemical reactions between hydrogen and oxygen present in the air.
When protons pass through the polymer membrane, the polymer membrane near to the anode is dried as a result of the migration of protons together with water molecules due to electro-osmosis. This drying process reduces the electrical conductivity of the polymer membrane and therefore has an adverse effect on power generation characteristics. The drying process may be prevented by moisturizing the hydrogen gas and air which are supplied to the electrodes. Tokkai 2001-256989 published by the Japanese Patent Office in 2001 discloses a humidifying device using water (or pure water) from a water tank in order to humidify supplied gases such as hydrogen and air. A polymer electrolyte fuel cell employing an external humidifying system is provided with a humidifying device for humidifying gases supplied to the fuel cell on an external section of the fuel cell. However when the fuel cell system is disposed in an external environment at a temperature of less than 0° C., it is not possible to humidify supplied gases such as hydrogen or air due to the fact that the water in the water tank freezes. Consequently, the fuel cell can only be started up after melting ice in the water tank. A prior art technique prevents freezing of water by using an electrical heater to heat the water tank or the humidifying device when the external temperature is less than 0° C.